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260 atures. No hatching occurred at go C. Our results differ from Marsh (1985) and Toney (1974) who found total egg mortality occurring at 10" C. Marsh (1985) also reported reduced hatching at 150.C. The development rates we observed were less than those reported by Minckley & Gustafson (1982) and Marsh (1985) at corresponding temperatures. This may have resulted from lower initial spawning and preacclimation temperatures of adults and eggs in our study. Our success in incubating eggs at lower temper- atures may reflect simulation of natural conditions to a greater degree than those of previous investi- gators. Marsh (1985) and Toney (1974) had higher acclimation temperature gradients (higher preac- climation temperatures). used formaldehyde to control fungus. and hormonally-induced matuTCI- tion of ova. Eggs used by Marsh (1985) were spawned at 17.5-18"C and those used by Toney (1974) were at 160C before being acclimated to lower temperatures. Different stages of embryonic development are more sensitive to both negative changes and the degree of change in temperature (Alabaster & Uoyd 1980. Cloud et at 19N8). In both studies. embryos were treated with formalde- hyde but the synergistic effects of formaldehyde and reduced temperatures are unknown, to us. Minckler & Gustafson (1982) found that \Wrmon- ally-inducefl ovulation resulted in razorback sucker eggs averaging 1.8 mm diameter. while naturally- ripened eggs averaged 2.9mm. Egg diameters in our experiments using natur-llly-ripened fish (mean = 3.1:t O.4sd) were similar to the latter. It is difficult to control all external variables and look solely at incubation response to temperature differences. Our use of naturally-ripc:ned adults. lower temperature acclimation. and avoidance of formaldehyde may have contributed to differences between our results and those of others. The varia- bility in hatching success among our experiments may indicate the presence of other factors influen- cing hatching success. Auctuations in temperature in the experimental chambers may have increased mortality at HrC. Temperature buffering in thc reservoir may reduce this source of mortality and therefore may not bc as significant in Lake Mo- have. IlIfluC'l/ce of temperature 011 reproductiol/ Temperature is an important regulator of metabo- lic and development rates in fish embryos (Win- berg 1956. Hoar & Randall 1969. Colby & Brooke 1973). Embryonic dcvelopment is fasler al higher temperatures. but is confined within some physio- logiC-ell minimum and maximum tcmperature that is specifIC to individual species. Optim..l rang~'S of embryonic incubation temperatures of many s~- cies tlf freshW'cder fish can vary. spanning a range of about geC or more (Alaba...tcr & Lloyd l\Jx()). Thc..-se optimal ranges are often concurrent with temperatures selected by that species in nature for spawning. Spawning also often occurs above and below these optimal temper.ltures. However. there is an 1ncreasc in mortalities and abnonmtlitics a...so- dated with incubation in these temperature ranges (Alabaster & Lloyd 19HO). Hatching success. largely influenced by temper- atures during the spawning season. can ultimately affc,-'1 year class strength offL'\h (Serns 1984. Reck- alln 11.)86, Coutant 1987. Kalkmeyn 1987). The wider r.mge of optimal incubation tempcmtures in freshwater fishes relative to marine fishes is be. lieved to bc an adaptation to the more variable thermal regimes occurring in freshwater systems (Alabaster & Lloyd 1980. Cushing 1982). Cushing (1~) further suggests that longer spawning peri- (xis by some spc..'Cies arc a Ilkochanism to reduce year class failure by having at least some larvae emerging during times of optimal environmental conditions. Razorback sucker-white suda~r spawni"g Linle is known about the pre-impounded riverine spawning. incubation. and ecology of the relzor- back sucker. particularly in regard to temperature. Reduced temperatures from hypolimnetic dis- charges of dams are implied as a contributing factor to lhe decline in distribution and abund.mce of the species (Johnson & Rinne 19H2. Marsh 1%5). However. the exact mechanism of this inter.lction is unknown. Comparison of incubation and rearing temperatures betwecn the r.lzorback sucker and